How much more is there to say about Margaret Thatcher? That these biographies have the same phrase in their title is not a promising start. Nor is it a title – taken from one of her most self-mythologising moments, her studiedly defiant speech to the doubting 1980 Conservative conference – that suggests these heavy volumes will be leavened with too much fresh or independent thinking. Robin Harris worked with Thatcher, often "closely" in his words, for a quarter of a century from the late 70s, as a speechwriter, ghostwriter, adviser, organiser and diehard supporter. In her memoirs, she calls him "my indispensable sherpa". Charles Moore has been one of Britain's best-known rightwing journalists for equally long. Since Thatcher's death, he has seemed happy to mix his promotional duties as an author with defending her against, as he put it on Question Time, "People who are horrible … promoting the idea that she is [sic] very divisive."

Moore was chosen by Thatcher to be her official biographer in 1997. It was the year her party finally lost power: her reputation, it was reasonable to assume, was going to need some protecting. "The arrangement that Lady Thatcher offered me," writes Moore, "was that I would have full access to herself … and to her papers. She would assist all my requests for interviews with others … As a result of her support … the then Cabinet Secretary, Sir Richard Wilson, gave permission for all existing and former civil servants to speak freely to me about the Thatcher years, and allowed me to inspect government papers, held back from public view under the thirty-year rule." Moore has exploited this unique access with thoroughness and skill; but a sense of the British establishment granting favours to one of its own hangs over this book, and is never quite dispelled.

Harris began his book in 2005, the year of another post-Thatcher Tory general election defeat. Despite the existence of the Moore project, it appears she was keen to collaborate with Harris too. He reprints a letter from her: "I can think of no one better placed than you to tackle the subject … You know, better than anyone else, what I wanted our reforms to achieve." Elsewhere in his preface, Harris pointedly describes Moore's book as "a further, 'authorized' work". As prime minister, one of Thatcher's ambitions was to make Britain more competitive; posthumously, it's clear that included her biographers.

Robin Harris, Not for Turning: The Life of Margaret Thatcher

Yet as in the utilities she privatised, competition sometimes doesn't produce much choice for consumers. Both these books begin, as almost every Thatcher biography has for decades, with a reverent depiction of her Grantham childhood, all formative hard graft and smalltown English virtues, which in the retelling – not least by Thatcher as a rising politician – has long become as sepia-tinted as a rustic Hovis advertisement. Moore describes her ambitious shopkeeper father as "tall, with piercing blue eyes and wiry blond hair"; Harris calls him "tall, blond and blue-eyed". As a young girl, writes Harris, Thatcher had "a sweet smile, beautiful hair, flashing blue eyes". Here, as in much of the rightwing writing about her since her death, Thatcher seems to be becoming a sort of Tory Evita.

But then Harris's book wakes up. In Grantham and afterwards, he abruptly remarks, Thatcher "would never be very interested in people's personalities … only in their actions – and specifically those of their actions that directly concerned her." Further tart assertions about her personality and habits quickly follow. When she ate, food would be "hoovered up as quickly as possible". When she worked on official papers as prime minister, she often sat "in her [Downing Street] study in a high-backed chair … Over the years her feet wore a hole in the carpet. She refused to have a new one and had a patch inserted."

In political conversation, "She had no real sense of place … adopting even in private discussion the same aggressive and self-justificatory stance as she would in a hostile television interview." As a thinker, although she carried a collection of excerpts from Winston Churchill's speeches and broadcasts in her handbag, Harris writes, she "did not have much historical sense, merely some rather romantic and fanciful historical notions".

After all the eulogies, it is refreshing to read about an odd, driven, believable person – rather than some abstract national saviour or demon. In his confident generalisations about Thatcher, Harris is like a long-faithful courtier freed by a monarch's death to speak the truth about them. He is not that interested in piling up evidence for his assertions. Like an article in the Spectator, the writing can be lordly rather than logical, and the word "probably" appears more often than in most biographies. Much of the book is closer to memoir or polemic – you need to take it on trust.

The recounting of Thatcher's dark-horse dash through the Conservative party pack and tumultuous premiership is efficient rather than revelatory. There are slow stretches where Harris summarises and justifies her policies, one by one; and equally relentless but more quotable attacks on Thatcher's many Tory enemies and allies-turned-nemeses, such as her chancellors Nigel Lawson ("too clever by half") and Geoffrey Howe ("raddled with bitterness").

Moore is more measured. His dense, intricate volume, the first of an intended two, follows Thatcher only up to the autumn of 1982, less than a third of the way into her premiership. For now at least, this cut-off date robs his version of her story of the always-compelling element of rise and fall – the latter vividly and emotionally depicted by Harris – and instead makes Moore's Thatcher narrative like one of the economic graphs in Thatcherism's boom years: jagged but generally upward.

There are some surprises, though. Thatcher's sister Muriel, barely mentioned by other biographers, is revealed as the recipient of frank letters from the teenage Margaret. Of an Oxford university boyfriend, pre-Denis, also previously undetected by biographers, she writes: "Tony hired a car and we drove out to Abingdon to the country inn 'Crown and Thistle'. I managed to borrow a glorious royal blue velvet cloak … I felt absolutely on top of the world as I walked through the lounge … and everyone looked up." That Thatcher had a bit of a life before parliamentary politics claimed her in the early 50s is a less sensational discovery than some of the publicity around this book has trumpeted; but Moore, with typical care and perceptiveness, produces a clever coda to his account of the Tony relationship. In 1974, long after it was over, Tony, now a stockbroker with a professional interest in the housing market, produced a scheme for council tenants to buy their homes. As the shadow minister responsible for housing, Thatcher invited him to the Commons. "She made only the most glancing acknowledgement of their old acquaintance and got straight down to the policy, towards which she was very receptive."

Margaret Thatcher when she was secretary of state for education and science at the Conservative party conference in Blackpool, 1972 Photograph: Jamie Hodgson/Getty Images

This is Moore's first book (Harris has written or ghostwritten half a dozen), and its prose is understated and less partisan than his journalism. Occasionally, the long, controlled paragraphs curl almost imperceptibly into dry wit. In the mid-60s, he writes, "At the highest levels of the [Tory] party … suspicions were aroused that the rise of Margaret Thatcher might represent some sort of threat to male peace and tranquility." Nor is Moore a total prisoner of his many sources. Their testimony is weighed, and sometimes contradicted. Even Muriel, who granted a rare interview, is corrected when she claims that Margaret was too busy to go to their father's funeral, with reference to Margaret's "two engagement diaries of the period" and a report in the Grantham Journal.

There is a downside to all this neat dovetailing of material and elegantly murmuring, High Tory style. Thatcherism was in many ways an unsubtle, unstable political project, exhilarating or brutal depending on where you stood; yet only the exhilaration feels fully present in Moore's narrative, for all his conscientious detailing of Thatcherism's 70s and 80s ups and downs. Part of the problem may be the slightly sketchy way he deals with the world beyond. There is not quite enough sense of the social texture of Britain, and how that changed, as Thatcher rose, and how that change helped her. Similarly, events outside Westminster that proved pivotal for her – the 1978-9 winter of discontent that probably won her the 1979 election; the 1981 urban riots that so undermined her early premiership – are recorded too briefly and cursorily. Meanwhile, Moore's politics surface unhelpfully when he caricatures postwar Britain as in "steep decline", the economy under Labour in the 60s as a "car crash", and the IMF that eagerly helped do away with British social democracy in the 70s as "impartial".

As much of the debate since her death has shown, there are still plenty of takers for this doomy, simplistic view of pre-Thatcherite Britain. But present-day historians are becoming steadily less keen on it, and the struggles of our Thatcherised economy since 2007 don't augur too well for the long-term reputation of books that present her rule as having solved all our problems. Moore is more nuanced than that; unlike Harris, he offers a few quiet but stinging criticisms of her policies, for example on council house sales, which led to "the gradual build-up of a housing shortage which, in 1979, had not existed, and the stoking, for the future, of a housing bubble".

The other long-term value of this book is likely to be its sheer quantity of new or rarely deployed material. Alfred Sherman, the fiercest of Thatcher's many thinkers, told Moore before he died: "She was a woman of beliefs, and beliefs are better than ideas." As the Falklands taskforce set off, the then French president François Mitterrand, in theory Thatcher's ideological opposite, mused aloud, according to the diary of one of his advisors, "Do I admire her … or envy her?" Journalists and academics will be combing these pages for quotes and details for years, as they did the previous big Thatcher biographies, John Campbell's two volumes from 2000 and 2003 and Hugo Young's early version from 1989. Compared with Young's compelling mix of admiration and castigation, and Campbell's panoramic, even-handed treatment, Moore's effort lacks originality sometimes, but he has more facts. They do not profoundly change the picture of Thatcher we already have, but facts about mythologised figures are valuable.

Few parts of her life are as hazy as her final years. In the best section of his book, Harris gives an extended, intimate account of the empty, sad quarter of a century that unfurled for her after the abrupt termination of her premiership and Tory leadership in 1990. The House of Lords "she found soporific" and "ponderous". The continuing hold of her ideas over the Conservative and Labour governments that followed hers did not compensate for the loss of office and the cross-party rejection of her governing style as too feverish and shrill. She looked for causes, not always wisely. When the former Chilean dictator Augusto Pinochet was arrested in London in 1998 for crimes against humanity, Thatcher campaigned for his release. Harris was one of the campaign's organisers, and reprises here his and Thatcher's rather chilling justifications for Pinochet's "beneficial" rule. Yet Harris also claims – in novelistic, unsparing detail – that when Thatcher visited Pinochet during his British confinement, he had recently suffered "a series of small strokes – as, indeed, had she, though she did not yet know it".

Another scene describes her working around this time in her office in wealthy, sterile Belgravia, in a "high-backed chair" as in Downing Street, "looking at papers with the help of a huge magnifying glass … underlining, marking marginalia, gathering odds and ends together in files, and then forgetting where she had put them". It will be interesting to see what Moore's second volume makes of her decline. It had fewer witnesses than her rise. Like the little-reported thinness of her funeral crowds once you got a few hundred yards away from St Paul's Cathedral, some sides of the Thatcher story are melancholy rather than inspiring, even for rightwingers. Now she's gone, perhaps biographers can be more honest about her limits.

• When the Lights Went Out: Britain in the Seventies by Andy Beckett is published by Faber